Atherosclerosis and vascular thrombosis are a major cause of morbidity and mortality, leading to coronary artery disease, myocardial infarction, and stroke. Atherosclerosis begins with an alteration in the endothelium, which lines the blood vessels. The endothelial alteration results in adherence of monocytes, which penetrate the endothelial lining and take up residence in the subintimal space between the endothelium and the vascular smooth muscle of the blood vessels. The monocytes absorb increasing amounts of cholesterol (largely in the form of oxidized or modified low-density lipoprotein) to form foam cells. Oxidized low-density lipoprotein (LDL) cholesterol alters the endothelium, and the underlying foam cells distort and eventually may even rupture through the endothelium.
Platelets adhere to the area of endothelial disruption and release a number of growth factors, including platelet derived growth factor (PDGF). PDGF, which is also released by foam cells and altered endothelial cells, stimulates migration and proliferation of vascular smooth muscle cells into the lesion. These smooth muscle cells release extracellular matrix (collagen and elastin) and the lesion continues to expand. Macrophages in the lesion elaborate proteases, and the resulting cell damage creates a necrotic core filled with cellular debris and lipid. The lesion is then referred to as a “complex lesion.” Rupture of this lesion can lead to thrombosis and occlusion of the blood vessel. In the case of a coronary artery, rupture of a complex lesion may precipitate a myocardial infarction, whereas in the case of a carotid artery, stroke may ensue.
One of the treatments that cardiologists and other interventionalists employ to reopen a blood vessel which is narrowed by plaque is balloon angioplasty (approximately 300,000 coronary and 100,000 peripheral angioplasties are performed annually). Although balloon angioplasty is successful in a high percentage of the cases in opening the vessel, it unfortunately denudes the endothelium and injures the vessel in the process. This damage causes the migration and proliferation of vascular smooth muscle cells of the blood vessel into the area of injury to form a lesion, known as myointimal hyperplasia or restenosis. This new lesion leads to a recurrence of symptoms within three to six months after the angioplasty in a significant proportion of patients (30-40%).
In atherosclerosis, thrombosis and restenosis there is also a loss of normal vascular function, such that vessels tend to constrict, rather than dilate. The excessive vasoconstriction of the vessel causes further narrowing of the vessel lumen, limiting blood flow. This can cause symptoms such as angina (if a heart artery is involved), or transient cerebral ischemia (i.e. a “small stroke”, if a brain vessel is involved). This abnormal vascular function (excessive vasoconstriction or inadequate vasodilation) occurs in other disease states as well. Hypertension (high blood pressure) is caused by excessive vasoconstriction, as well as thickening, of the vessel wall, particularly in the smaller vessels of the circulation. This process may affect the lung vessels as well causing pulmonary (lung) hypertension. Other disorders known to be associated with excessive vasoconstriction, or inadequate vasodilation include transplant atherosclerosis, congestive heart failure, toxemia of pregnancy, Raynaud's phenomenon, Prinzmetal's angina (coronary vasospasm), cerebral vasospasm, hemolytic-uremia and impotence.
Because of their great prevalence and serious consequences, it is critically important to find therapies which can diminish the incidence of atherosclerosis, vascular thrombosis, restenosis, and these other disorders characterized by abnormality of vascular function and structure. Ideally, such therapies would inhibit the pathological vascular processes associated with these disorders, thereby providing prophylaxis, retarding the progression of the degenerative process, and restoring normal vasodilation.
As briefly summarized above, these pathological processes are extremely complex, involving a variety of different cells which undergo changes in their character, composition, and activity, as well as in the nature of the factors which they secrete and the receptors that are up- or down-regulated. A substance released by the endothelium, “endothelium derived relaxing factor” (EDRF), may play an important role in inhibiting these pathologic processes. EDRF is now known to be nitric oxide (NO) or a labile nitroso compound which liberates NO. (For purposes of the subject invention, unless otherwise indicated, nitric oxide (NO) shall intend nitric oxide or the labile nitroso compound which liberates NO.) This substance relaxes vascular smooth muscle, inhibits platelet aggregation, inhibits mitogenesis and proliferation of cultured vascular smooth muscle, and leukocyte adherence. Because NO is the most potent endogenous vasodilator, and because it is largely responsible for exercise-induced vasodilation in the conduit arteries, enhancement of NO synthesis could also improve exercise capacity in normal individuals and those with vascular disease. NO may have other effects, either direct or indirect, on the various cells associated with vascular, walls and degenerative diseases of the vessel.